1. Field of the Invention
The present invention relates to an antenna mounted on and retractable into the casing of a portable radio communication apparatus and, more particularly, to an antenna having a sufficient gain even when retracted into the casing.
2. Description of the Related Art
It is a common practice with an antenna for the above application to use a half wavelength (.lambda./2) whip antenna whose gain is hardly susceptible to the human body, thereby insuring high quality conversations. However, if the .lambda./2 whip antenna is exposed to the outside at all times, it constitutes an obstruction and degrades the portability or mobility. In the light of this, it has been customary to allow the whip antenna to be retracted into the casing of the apparatus, as needed. This, however, brings about another problem that when the antenna is received in the casing, the antenna gain is noticeably lowered. Hence, when the user of the apparatus carries it while waiting for a call, the antenna has to be held in an extended position, also degrading the portability.
Japanese Utility Model Laid-Open Publication No. 61-57608, for example, teaches a rod antenna having a coil-like portion at the tip thereof, and a cover made of synthetic resin and covering the coil-like portion. The problem with this kind of antenna is that when it is applied to a portable radio communication apparatus and received in the casing of the apparatus, it cannot have the same gain as when pulled out of the casing.